The present invention relates to an improved igniter plug that will withstand long term use at high temperatures. Generally, igniter plugs are used in engines, such as aircraft engines, to provide the spark for ignition of fuel. Such plugs comprise a central electrode that is composed of tungsten or other conductive material and an outer annular electrode spaced therefrom, to provide an air gap, with conductivity established between the center electrode and an electrical source. The annular electrode generally serves as a ground. A ceramic insulator is provided in the air gap. Upon energizing of the center electrode, a spark is produced across the gap between the center electrode and the annular electrode, which spark ignites the fuel for use in the engine.
The use of tungsten as the material from which the center electrode is formed has been preferred. However, due to the high temperature and environment to which the electrode is subjected during operation of the igniter plug, tungsten oxide is sometimes formed on the electrode surface which causes the electrode surface to expand and exercise forces on the initially spaced ceramic insulator, causing cracks in the insulator top. Such cracks shorten the effective life of the igniter plug.
The problem posed by formation of an oxide film on the surface of the tungsten tip insulator is described in U.S. Pat. No. 3,890,518 wherein an oxidation resistant material, such as silicon or aluminum, is provided over the tungsten part of the inner electrode as a diffused surface layer. The application of the silicon or aluminum coating is effected by surrounding the entire electrode tip with a fine powder of the desired material and heating the assembly for four hours at a temperature of 1060.degree. C. under an argon atmosphere. In that embodiment, the electrode tip is provided with an oxidation resistant layer of diffused silicon by coating the complete tip of a thickness of about 15 microns and the layer is removed from the axial end surface of the tip that is to be connected to the main electrode or pin.